After being reviled for almost 2,000 years as the embodiment of treachery, Judas Iscariot's side of the story was finally published yesterday. Thanks to a newly discovered gospel in Judas's name, we now know what his excuse was: Jesus made me do it.

The Gospel of Judas, a fragile clutch of a leather-bound papyrus thought to have been inscribed in about AD300, was unveiled yesterday in Washington by the National Geographic Society, and it represents a radical makeover for one of the worst reputations in history.

According to this version of events, not only was Judas obeying orders when he handed Jesus to his persecutors, he was Christ's most trusted disciple, singled out to receive mystical knowledge.

According to the 26-page gospel, copied in the ancient Coptic language apparently from a Greek original more than a hundred years older, Jesus told Judas: "Step away from the others and I shall tell you the mysteries of the kingdom. It is possible for you to reach it, but you will grieve a great deal."

In the days before the fateful Passover holiday, Jesus also told Judas: "You will exceed all of them. For you will sacrifice the man that clothes me." The line, according to biblical scholars, suggests that Jesus chose Judas to help him achieve his destiny by liberating him from his earthly body.

"It's a striking contrast with the negative portrayal of Judas as the quintessential traitor," said Marvin Meyer, a biblical scholar from Chapman University in California who helped translate the gospel. "The figure of Judas is often portrayed as the evil Jewish person who turned Jesus in to be killed."

It is unlikely, however, that the documents are about to trigger a total rehabilitation for the Iscariot name, with shrines in his name and readings from his gospel at church services, let alone a film treatment by Mel Gibson.

The initial reaction from Christian scholars was wary. Even if the gospel is authentic, they said, it appears to be the work of a particular 2nd-century sect, the gnostics, who had different beliefs from the mainstream church and who were long ago declared heretical.

The leading biblical scholar and translator of the dead sea scrolls, Professor Geza Vermes of Oxford University, said: "The document is of interest for the ideas of the gnostics but it almost certainly adds nothing to our understanding of what happened 150 years before it was written."

But the Gospel of Judas is bound to focus more attention on the gnostics, whose belief that they possessed secret knowledge leading to salvation resonates with new-age mysticism. Another gnostic text, the Gospel of Mary Magdalene, discovered a century ago, has been given a dose of publicity lately by The Da Vinci Code.

The manuscript also serves as a reminder that the four gospels in the New Testament were not the only versions of Jesus's life in the early Christian era, according to Bart Ehrman, a religious studies professor at the University of North Carolina, at its unveiling yesterday. "In the struggle among Christian groups to win converts only one emerged victorious," he said. "It declared itself orthodox and all others heretics."

The Gospel of Judas is known to have existed before AD180, when it was denounced as heretical by Bishop Irenaeus of Lyon. But it was thought to have been lost when the gnostics were vanquished in the struggle of ideas in the early years of Christianity.

The papyrus manuscript, also known as a codex, was found in an Egyptian cave in 1978. It circulated among antiquities traders for a while before it was locked in a safe deposit box in Long Island, New York, by a collector. It was bought in 2000 by a Swiss dealer, who realised its importance and its rapidly deteriorating condition and handed it over to the Maecenas Foundation for Ancient Art in Basel the next year.

"The manuscript was so brittle, it would crumble at the slightest touch," said Rodolphe Kasser, a Coptic expert called in to help reconstruct it.

It has taken five years to reassemble the fragments, translate the manuscript and authenticate it. Radiocarbon dating by the University of Arizona showed it was likely to have been written between AD220 and 340.

Scholars in Britain pointed out that the document appeared to have been written several decades after the last of the gospels in the New Testament - that of John - and at least a century after the death of Judas Iscariot.

The Rt Rev John Pritchard, the Church of England Bishop of Jarrow, said: "I don't think it changes anything about the truth of the Christian faith. There has long been a theory that Judas wanted to flush Jesus out, to declare a rebellion and drive the Romans out and killed himself when he realised he had got it wrong, but we don't know that and neither did whoever wrote this document decades after the event. This is not something to put your faith on."

By the time the Gospel of Judas was written the basic content of the New Testament was in place; the criteria for choosing the writings of Matthew, Mark, Luke and John, as opposed to any of up to 30 alternatives, were their authenticity and direct authorial links to men who knew Jesus. Other texts, including presumably this one, were sifted out over succeeding centuries.

According to the Gospel of Judas, Jesus tells him: "You will be cursed by the other generations - and you will come to rule over them."

The first half of that prophecy has certainly come to pass. The second, even after yesterday's revelations, still seems a long way off.

· The Gospel of Judas will be shown on the National Geographic Channel on Sunday at 9pm